Before discussing why you would want to pin a cluster it would be useful to understand the different states of a cluster. We can have:
I usually have “auto terminate” on meaning that after 2 hours I would like to cluster shut down to save costs, however 30 days after a cluster is terminated, it is permanently deleted. You might not want this. If so, this is where you PIN the cluster.
How? See that pin in the blue circle below? Click it